The degree of oxygen saturation of hemoglobin, SpO.sub.2, in arterial blood is often a vital index of the condition of a patient. As blood is pulsed through the lungs by the heart action, a certain percentage of the deoxyhemoglobin, RHb, picks up oxygen so as to become oxyhemoglobin, HbO.sub.2. From the lungs, the blood passes through the arterial system until it reaches the capillaries at which point a portion of the HbO.sub.2 gives up its oxygen to support the life processes in adjacent cells.
By medical definition, the oxygen saturation level is the percentage of HbO.sub.2 over the total hemoglobin; therefore, SpO.sub.2 =HbO.sub.2 /(RHb+HbO.sub.2). The saturation value is a very important physiological value. A healthy, conscious person will have an oxygen saturation of approximately 96 to 98%. A person can lose consciousness or suffer permanent brain damage if that person's oxygen saturation value falls to very low levels for extended periods of time. Because of the importance of the oxygen saturation value, "Pulse oximetry has been recommended as a standard of care for every general anesthetic." Kevin K. Tremper & Steven J. Barker, Pulse Oximetry, Anesthesiology, January 1989, at 98.
An oximeter determines the saturation value by analyzing the change in color of the blood. When radiant energy passes through a liquid, certain wavelengths may be selectively absorbed by particles which are dissolved therein. For a given path length that the light traverses through the liquid, Beer's law (the Beer-Lambert or Bouguer-Beer relation) indicates that the relative reduction in radiation power (P/Po) at a given wavelength is an inverse logarithmic function of the concentration of the solute in the liquid that absorbs that wavelength.
For a solution of oxygenated human hemoglobin, the absorption maximum is at a wavelength of about 640 nanometers (red), therefore, instruments that measure absorption at this wavelength are capable of delivering clinically useful information as to oxyhemoglobin levels.
In general, methods for noninvasively measuring oxygen saturation in arterial blood utilize the relative difference between the electromagnetic radiation absorption coefficient of deoxyhemoglobin, RHb, and that of oxyhemoglobin, HbO.sub.2. The electromagnetic radiation absorption coefficients of RHb and HbO.sub.2 are characteristically tied to the wavelength of the electromagnetic radiation traveling through them.
It is well known that deoxyhemoglobin molecules absorb more red light than oxyhemoglobin molecules, and that absorption of infrared electromagnetic radiation is not affected by the presence of oxygen in the hemoglobin molecules. Thus, both RHb and HbO.sub.2 absorb electromagnetic radiation having a wavelength in the infrared (IR) region to approximately the same degree; however, in the visible region, the light absorption coefficient for RHb is quite different from the light absorption coefficient of HbO.sub.2 because HbO.sub.2 absorbs significantly more light in the visible spectrum than RHb.
In practice of the pulse oximetry technique, the oxygen saturation of hemoglobin in intravascular blood is determined by (1) alternatively illuminating a volume of intravascular blood with electromagnetic radiation of two or more selected wavelengths, e.g., a red wavelength and an infrared wavelength, (2) detecting the time-varying electromagnetic radiation intensity transmitted through or reflected back by the intravascular blood for each of the wavelengths, and (3) calculating oxygen saturation values for the patient's blood by applying the Lambert-Beer's transmittance law to the detected transmitted or reflected electromagnetic radiation intensities at the selected wavelengths.
Whereas apparatus is available for making accurate measurements on a sample of blood in a cuvette, it is not always possible or desirable to withdraw blood from a patient, and it obviously impracticable to do so when continuous monitoring is required, such as while the patient is in surgery. Therefore, much effort has been expanded in devising an instrument for making the measurement by noninvasive means.
The pulse oximeters used today are desk-top models or handheld models that are interfaced to the patient through the use of a multi-wire bundle. Despite their size and level of technology, these units are still bound by several limitations.
A critical limitation is that of measurement accuracy. In pulse oximetry, signal artifact from patient-probe motion, ambient light, and low perfusion (low blood circulation through the extremities) is one of the primary causes of inaccurate saturation readings. ("Artifact" is any component of a signal that is extraneous to variable represented by the signal.) Inaccuracies are also caused from physiologic nonlinearities and the heuristic methods used to arrive at the final saturation values.
Another important limitation is patient confinement to the pulse oximeter, due to the wired probe connecting the patient to the unit. This limits patient mobility in every application of its use, including the emergency room, operating room, intensive care unit, and patient ward.
Thus, three problems plague pulse oximetry. The first problem relates to signal artifact management and inaccuracies of the saturation values due to the nonlinear nature of the sample tissue bed. The second problem relates to noise from signal artifact which introduces further inaccuracies. The third problem relates to restricted patient mobility and probe placement due to the wire bundle that physically couples the patient to the oximeter unit and the exclusive use of transmittance-type probes.
Due to the non-linear nature of human physiology, engineers were forced to employ techniques for calculating the final saturation value based not on an analytic solution, but rather, on a calibration curve or look-up table derived from empirical data. This is data that has been collected over hundreds or possibly thousands of patients and stored as a look-up table in the system memory. This technique leads to obvious inaccuracies in the final saturation value since the SpO.sub.2 value in the look-up table is only as accurate as the calibration curve programmed into the system memory, which in turn is only as accurate as the in vitro laboratory oximeter used to generate it. These inaccuracies are compounded by differences in skin characteristics between patients, as well as differences over the skin surface of the same patient.
Signal artifact has three major sources: (1) ambient light (which causes an AC/DC masking signal), (2) low perfusion (in which the intensity of the desired AC/DC signal is very low thereby allowing other artifact sources to mask the desired signal more easily), and (3) patient or sensor motion (which generates a large AC/DC artifact masking the desired signal). When the oximetry signal is amplified, the noise components are amplified along with the desired signal. This noise acts to corrupt the primary signal, during both pre-processing as well as post-processing, thereby reducing the accuracy of the pulse oximeter reading. Signal artifact is prevalent with both reflectance- and transmittance-type probes.
Restricted patient mobility is due to the hard wired interface that links the patient probe to the large, bulky oximeter unit. This link is a multi-wire bundle that is used to provide an electrical path for the LED drivers and the photodiode located at the end of the wire bundle in the probe. Probes employing transmittance-type method are restricted to the ears, fingers, or toes and, thus, require physical access to these areas exclusively.
Oximeters are large because of the circuitry heretofore believed necessary to capture the signals and because such higher-powered circuitry shortens battery life. Typical digital oximeters use a silicon photodiode, a current-to-voltage converter (a transimpedance amplifier), a preamplifier, filter stage, a sample and hold, and an analog-to-digital (A/D) converter to capture the oximetry signal. These components make the creation of truly portable oximeters difficult because of the large footprint and high power requirements of each device. The A/D converter, in particular, is typically large and power-hungry.